The increase in Internet traffic and other telecommunications over the past several years has caused researchers to explore new ways to increase fiber optic network capacity by carrying multiple data signals concurrently through telecommunications lines. To expand fiber network capacity, fairly complex optical components have already been developed for wavelength division multiplexing (WDM) and dense wavelength division multiplexing (DWDM).
In a WDM system, multiple optical data signals of different wavelengths are added together in a device called a multiplexer and the resulting data signal is transmitted over a fiber optic cable. The wavelength division multiplexed signal comprises a plurality of optical signals having a predetermined nominal wavelength difference from each other. A demultiplexer separates the multiple optical data signals of different wavelength. Any WDM system must include at least one component to perform the function of optical multiplexing (namely, the multiplexer) and at least one component to perform the function of optical demultiplexing (namely, the demultiplexer). The optical multiplexer and the optical demultiplexer are each examples of optical wavelength routers.
In general, an optical wavelength router has at least one input optical port and at least one output optical port. In an optical router, light may be transmitted from a specific input port to a specific output port only if the light has an appropriate wavelength. Complex WDM systems may require optical wavelength router components that are more complex than a multiplexer or a demultiplexer.
Planar lightwave circuit technology is one technology that may be used to implement an optical wavelength router. A planar lightwave circuit (PLC) is an application of integrated optics. In a PLC, light is restricted to propagate in a region that is thin (typically between approximately 1 μm and 30 μm) in one dimension, referred to herein as the lateral dimension, and extended (typically between 1 mm and 100 mm) in the other two dimensions. The plane in which the PLC is disposed is defined as the plane of the PLC. The longitudinal direction is defined as the direction of propagation of light at any point on the PLC. The lateral direction is defined to be perpendicular to the plane of the PLC. The transverse direction is defined to be perpendicular to both the longitudinal and the lateral directions.
In a typical example of a PLC, a slab waveguide comprises three layers of silica glass, a core layer lying between a top cladding layer and a bottom cladding layer. Channel waveguides are often formed by at least partially removing (typically with an etching process) core material beyond the transverse limits of the channel waveguide and replacing it with at least one layer of side cladding material that has an index of refraction that is lower than that of the core material. The side cladding material is usually the same material as the top cladding material. In this example, each layer is doped in a manner such that the core layer has a higher index of refraction than either the top cladding or bottom cladding. When layers of silica glass are used for the optical layers, the layers are typically deposited on a silicon wafer. Deposition processes may include, chemical vapor deposition (CVD), low pressure chemical vapor deposition (LP-CVD), and/or plasma-enhanced CVD (PECVD). As a second example, slab waveguides and channel waveguides comprise three or more layers of InGaAsP. In this example, adjacent layers have compositions with different percentages of the constituent elements In, P, Ga, and As. As a third example, one or more of the optical layers of the slab waveguide and/or channel waveguide may comprise an optically transparent polymer. In this example, spin coating is one known film deposition method. Another example of a slab waveguide comprises a layer with a graded index such that the region of highest index of refraction is bounded by regions of lower indices of refraction. Graded index structures are commonly formed by dopant in-diffusion and have been used for LiNbO3 waveguides. A doped-silica waveguide is usually preferred because it has a number of attractive properties including low cost, low loss, low birefringence, stability, and compatibility for coupling to fiber.
The arrayed-waveguide grating router (AWGR) is the preferred integrated optical router. An AWGR is a planar lightwave circuit comprising at least one input channel waveguide, an input planar waveguide, an arrayed-waveguide grating (AWG), an output planar waveguide, and at least one output channel waveguide. The edge of the input planar waveguide to which the input channel waveguides are attached is referred to herein as the input focal curve. The edge of the output planar waveguide to which the output channel waveguides are attached is referred to herein as the output focal curve. The arrayed-waveguide grating comprises an array of channel waveguides. The length of the ith waveguide in the AWG is denoted as Li. The angular dispersion that is provided by the AWG is determined in part by the difference in length between adjacent waveguides, Li+1−Li. The details of construction and operation of the AWGR are described in K. Okamoto, Fundamentals of Optical Waveguides, pp. 346-381, Academic Press, San Diego, Calif., USA (2000). Each of the publications and patents referred to in this application are herein incorporated by reference in their entirety.
FIG. 1A depicts a conventional AWG router (AWGR) that acts as a demultiplexer 10. A plurality of optical signals incident on one input optical port propagates through the device in the following sequence: the signals propagate through an input waveguide 12, which is a input waveguide associated with the input port; through an input slab waveguide 14, which has the function of expanding the optical field in the transverse direction by diffraction; through the dispersive region 16 (namely, the array waveguide region) comprising an array of AWG waveguides 18 for modifying the direction of propagation for each wavelength constituent according to the wavelength of the constituent of the plurality of signals; through an output slab waveguide 20 for focusing the signals of different wavelength coupled from the dispersive region 16 into a plurality of predetermined positions in accordance with the predetermined wavelength difference; through a plurality of output waveguides 22 each associated with one output port. FIG. 1A depicts an AWG comprising six waveguide; however, any number of waveguides may be used and herein the number of waveguides used is referred to as “N.” A representative cross-section 30, section 1B—1B, of waveguide gratings 16 from FIG. 1A is shown in FIG. 1B. Depicted are the substrate 34, the bottom cladding 36, the top cladding 38, and waveguides comprising core material 31, 32, 33. These waveguides are typically buried channel waveguides as shown and typically have a core region with uniform height and width as seen in first, intermediate, and Nth waveguides 31, 32, 33, respectively. That is, the height of each waveguide of the grating is identical and the width of each waveguide of the grating is identical.
The dispersive property of the arrayed waveguide grating (AWG) region is attributable to the construction of the plurality of waveguides within the waveguide grating region such that adjacent waveguides have a predetermined length difference in accordance to the required dispersive properties of the dispersive region 16, so that each signal at different wavelength coupled to and traveling over each channel waveguide 18 is provided with a phase difference from each other in accordance with the predetermined length difference. Each of the output waveguides 22 includes an input end 24, which is arranged at a predetermined position, so that each separated signal at each wavelength is coupled to each output waveguide 22 and emerges from an output end 26 thereof.
In operation, the wavelength division multiplexed signals coupled into the input channel waveguide 12 expand into the input slab waveguide 14 by diffraction. Then, the expanded signals are distributed to the channel waveguides 18 of the arrayed-waveguide grating 16. Because each channel waveguide 18 of the arrayed-waveguide grating 16 has a predetermined waveguide length difference, each signal, after traveling over each channel waveguide 18 to the output slab waveguide 20, has a predetermined phase difference according to its waveguide length difference. Since the phase difference depends on the wavelength of the signal, each signal at different wavelength is focused on a different position along the arc boundary 28 of the output slab waveguide 20. As a result, separated signals, each having a different wavelength, are received by the plurality of output channel waveguides 22 and emerge therefrom, respectively.
The general principles and performance of an AWGR multiplexer are similar to the AWGR demultiplexer, except that the direction of propagation of light is reversed, the ports that act as inputs for the demultiplexer act as output ports for the multiplexer, and the ports that act as output ports for the demultiplexer act as input ports for the multiplexer.
Alternatively, an AWGR may comprise a plurality of output waveguides and a plurality of input waveguides; however, the general principles and performance are similar to the AWGR demultiplexer
Multiple routing functions including multiplexing and demultiplexing may be integrated on a silicon wafer to form a complex planar lightwave circuit (PLC). PLCs can be made using tools and techniques developed to extremely high levels by the semiconductor industry. Integrating multiple components on a PLC may reduce the manufacturing, packaging, and assembly costs per function.
One aspect of performance that is affected by the present invention is referred to as polarization dependent wavelength (PDW). This term, as well as a number of related terms, will now be defined. Spectral transmissivity (in units of dB) is defined as the optical power (in units of dBm) of substantially monochromatic light that emerges from the fiber that is coupled to the input port minus the optical power (in units of dBm) of the light that enters the optical fiber that is coupled to the output port of the optical router. Spectral transmissivity is a function of the selected input port, the selected output port, the optical wavelength, and the polarization state of the incident light. When the incident light is in a polarization state called a “principle state of polarization,” the light will be in the same polarization state when it emerges from the device. For purposes of illustration only, the principle states of polarization are assumed to be independent of wavelength, input port and output port. It is understood that the invention is not so limited by this assumption. Again, for the purposes of illustration only, it will be assumed that the two principle states of polarization are the so-called transverse electric (TE) and transverse magnetic (TM) polarization states. The TE polarization state has an electric field that is predominantly aligned in the transverse direction and the TM polarization state has an electric field that is predominantly aligned in the lateral direction. Again, the invention is not so limited to devices having these principle states of polarization. Typically, the device performance is sensitive to the polarization state of the incident light is attributable to birefringence in the planar waveguides and the channels waveguides comprising the AWGR.
FIG. 2A depicts, for a particular input/output port combination, a first spectral transmissivity 40 associated with the TE polarization state and a second spectral transmissivity 42 associated with the TM polarization state. Typically, for values of spectral transmissivity that are larger than −10 dB, the TM spectral transmissivity is a replica of the TE spectral transmissivity that is shifted in wavelength by an amount that is referred to as the polarization dependent dispersion (PDD). Herein PDD is positive if the TM spectral transmissivity has a maximum that has a longer wavelength than the maximum of the TE spectral transmissivity and is negative otherwise. Polarization dependent wavelength (PDW) is defined herein as the absolute value of the PDD and is indicated in FIG. 2A. The curves for the spectral transmissivity 46, 48, 50, 52 for four input/output combinations are shown together in FIG. 2C. The absolute value of the difference between the spectral transmissivities for TE and TM polarization states is referred to as the spectral polarization dependent loss 44 and is depicted in FIG. 2B. The in-band PDL (IB-PDL) is the maximum value of the spectral polarization dependent loss within a specified wavelength range called a “band” (typically a 0.2 nm range) for a particular input port and output port. The PDL for the device is typically defined as the largest value of IB-PDL among the values of IB-PDL for all input/output port combinations that are used in a particular application. To meet typical application requirements, it is critical for AWGRs to have a PDL value that is as close to 0 dB as possible.
In typical fiber optic communication systems, the polarization state of the light in the optical fiber may change in a manner that is uncontrolled and unpredictable. A change in the polarization state of the light in the fiber as it enters an AWGR will cause a change in the optical power that emerges from the AWGR that may be as large as the value of PDL for the AWGR. Because applications typically have little tolerance for such unpredictable changes in power, minimizing the PDL of an AWGR is highly desirable. PDL can be minimized by minimizing PDW. To meet typical requirements, PDW may be required to be less than 0.05 nm. For this reason, the design and manufacture of an AWGR that has a low value of PDW is highly desirable, yet very challenging.
There have been a number of techniques developed in an attempt to minimize PDW.
One approach to minimizing PDW involves selecting an optical layer design with minimum birefringence. In one example of this approach, U.S. Pat. No. 5,930,439 (Ojha et al) discloses a planar optical waveguide which reduces birefringence by doping the various optical layers so that the top cladding has a thermal coefficient of expansion that is close to the thermal expansion coefficient of the substrate. This approach is appropriate for an optical layer design comprising deposited silica layers with high concentrations of boron on a silicon substrate. Typically, this approach is impractical because the optical layers that are required for low birefringence are not capable of surviving standard reliability tests. For example, the optical layers may absorb water and subsequently form defects during a reliability test involving exposure to a temperature of 85° C. and a relative humidity of 85%,
A second approach requires the introduction of an optical waveplate. For example, U.S. Pat. No. 5,901,259 (Ando et al.) teaches forming an optical waveplate by using a polyimide having a film thickness of 20 μm or smaller and further teaches the introduction of the waveplate onto an AWGR to reduce PDW. However, introducing a waveplate onto the AWGR typically reduces the performance of the AWGR with respect to insertion loss, directivity, and return loss and occasionally may cause the AWGR to break. Furthermore, the introduction of a waveplate increases the cost associated with the production of the AWGR.
A third approach to reducing PDW involves waveguides of the AWG that comprise three segments, a central segment and two flanking segments. A first flanking segment has a birefringence equal to that to the second flanking segment. The central segment has a birefringence that is different from the flanking segments. The boundary around the central waveguide segments defines a region that is referred to herein as a “patch.” By selecting lengths of the segments that are appropriate to values of birefringence of the segments, an AWGR can be realized with a small value for PDW. A variety of methods have been disclosed for providing for segments with differing values of birefringence. For example, C. G. M. Vreeburg, et al. in “A low-loss 16-channel polarization dispersion-compensated PHASAR demultiplexer,” IEEE Photonics Technology Letters, Vol. 10, No. 3, Pp. 382-384 (1998) discloses a method wherein the AWG comprises InP-based rib waveguides, and the central segment differs from the flanking segments with respect to width of the rib and thickness of the top cladding region above the rib. In general, waveguides may have birefringence contributions from two independent sources, namely, form birefringence and stress birefringence. For rib waveguides, changing the width of the waveguide changes the form birefringence but does not substantially change the stress birefringence. For buried channel waveguide, changing the width of the waveguide does not substantially change the form birefringence. The effect of the width of a buried channel waveguide on the value of stress birefringence in the waveguide is not well known in the prior art.
In a second example, U.S. Pat. No. 5,341,444 (Henry et al.) discloses a method that includes the deposition of a high index material, such as silicon nitride, in the patch region so that it is optically coupled to the waveguide segments below it and thereby provides the central segments with a birefringence that is different from the birefringence of the flanking segments.
In a third example, U.S. Pat. No. 5,623,571 (Chou et al.) discloses a method that includes reducing the thickness of the cladding material in the patch region so that waveguide segments below couple to the air above the top cladding in the patch region and thereby provide the central segments with a birefringence that is different from the birefringence of the flanking segments.
In a fourth example, C. K. Nadler et al. in “Polarization Insensitive, Low-Loss, Low-Crosstalk Wavelength Multiplexer Modules,” IEEE Journal of Selected Topics in Quantum Electronics, Vol. 5, No. 5, pp. 1407-1412 (1999), discloses a method for compensating polarization sensitivity of AWGs by using “stress release” grooves etched on each side of the grating waveguide in the central region.
In all of these examples of this approach, extra process steps are required to provide the waveguide segments within the patch region with a birefringence that is different from the flanking waveguide segments. The disclosed methods are difficult to implement in practice because production of the required optical layers within the patch region within the required tolerances is difficult. The added complexity associated with the production of two different optical layer designs in two different regions also increases the cost of production. Despite the approaches above, PDW remains a problem in current AWGR designs.